In a conventional radiographic system an X-ray radiograph is obtained by X-rays transmitted imagewise through an object and converted into light of corresponding intensity in a so-called intensifying screen (X-ray conversion screen) wherein phosphor particles absorb the transmitted X-rays and convert them into visible light and/or ultraviolet radiation to which a photographic film is more sensitive than to the direct impact of X-rays.
In practice the light emitted imagewise by said screen irradiates a contacting photographic silver halide emulsion layer film which after exposure is developed to form therein a silver image in conformity with the X-ray image.
For use in common medical radiography the X-ray film comprises a transparent film support double-side coated with a silver halide emulsion layer. During the X-ray irradiation said film is arranged in a cassette between two X-ray conversion screens each of them making contact with its corresponding silver halide emulsion layer.
Single-side coated silver halide emulsion films combined in contact with only one screen are often used in autoradiography and to improve image definition which is of great importance, e.g. in mammography and in particular fields of non-destructive testing (NDT) known as industrial radiography. An autoradiograph is a photographic record formed through the intermediary of penetrating radiation emitted by radioactive material contained in an object, e.g. microtome cut for biochemical research.
Phosphors suited for use in the conventional radiographic system must have a high prompt emission on X-ray irradiation and low afterglow in favour of image sharpness.
More recently (see e.g. U.S. Pat. No. 3,859,527 and the periodical Radiology, Sept. 1983) an X-ray recording system has been developed wherein photodestimulable storage phosphors are used having in addition to their immediate light emission (prompt emission) on X-ray irradiation the property to store temporarily a large part of the energy of the X-ray image which energy is set free by photostimulation in the form of light different in wavelength characteristic from the light used in the photostimulation. In said X-ray recording system the light emitted on photostimulation is detected photoelectronically and transformed into sequential electrical signals.
The basic constituents of such X-ray imaging system operating with storage phosphors are an imaging sensor containing said phosphor, normally a plate or panel, which temporarily stores the X-ray energy pattern, a scanning laser beam for photostimulation, a photoelectronic light detector providing analogue signals that are converted subsequently into digital time-series signals, normally a digital image processor which manipulates the image digitally, a signal recorder, e.g. magnetic disk or tape, and an image recorder for modulated light exposure of a photographic film or an electronic signal display unit, e.g. cathode-ray tube.
The terminology X-ray conversion screen as used herein refers to screens for use in conventional screen-film combinations or for use in stimulated luminescence radiography.
From the preceding description of said two X-ray recording systems operating with X-ray conversion phosphor screens in the form of a plate or panel it is clear that said plates or panels serve only as intermediate imaging elements and do not form the final record. The final image is or reproduced on a separate recording medium or display. The phosphor plates or sheets can be used repeatedly. Before re-use of the photostimulable phosphor panels or sheets, a residual energy pattern is erased by flooding with light. The expected life of the plate is limited mainly by mechanical damage.
Common X-ray conversion screens comprise in order:a support, a layer comprising phosphor particles dispersed in a suitable binder and a transparent protective coating applied to the phosphor containing layer to protect said layer during use.
Since in the above-described X-ray recording systems the X-ray conversion screens are used repeatedly, it is important to provide them with an adequate topcoat for protecting the phosphor-containing layer from mechanical and chemical damage. This is particularly important for photostimulable radiographic screens where each screen normally is not encased in a cassette but is transported and handled as such.
Further, the lifetime of the radiographic screen is markedly prolonged by additional protection of the side surfaces of the screen, so-called edge-reinforcement, giving adequate protection against mechanical shocks occuring in transport outside a cassette. Particularly when operating storage phosphor screens in a continuous cycle of exposing the stimulable phosphor screen to radiation, reading out the radiation image recorded therein, and removing the remaining radiation energy the screen has to be moved through various processing stations, normally by roller transport. The repeated introduction of the screens between the nip of the transport rollers produces mechanical shocks that in the longer term cause damages resulting in abrasion and/or stripping of the phosphor containing layer. Edge reinforcement offers a solution to that problem.
The edge reinforcing coating should exhibit good adhesive properties towards chemically quite distinct materials; (1) the support material, e.g. polyester, (2) the phosphor layer composition comprising inorganic phosphor pigments bond by a small amount of an organic binder medium, and optionally the material of a protective topcoat. By poor adhesion to one or more of these elements the benefit of the inherent high abrasion resistance and mechanical strength of an edge reinforcing coating may be lost.
Edge-reinforcement of phosphor screens, particularly stimulable phosphor screens has been described e.g. in U.S. Pat. No. 4,788,435 according to which the side surfaces of a phosphor screen panel are covered with a polymer material composed of a linear polyester or a mixture of a linear polyester and a vinyl chloride-vinyl acetate copolymer.
According to said prior art the edge-reinforcement is performed by dissolving the polymer material in a relatively large amount of solvent of the aromatic type (toluene) and applying the solution to the edge faces of the screen and then drying the coating of the solution. Where possible the use of aromatic (toxic and inflammable) solvents should be avoided for ecological reasons.